A conventional toothed belt includes core wires, that is, stretching members formed at the center portion thereof, a tooth portion and a back portion. The tooth portion is formed of a tooth rubber, and the back portion is formed of a back rubber. Assuming that the side on which teeth are formed is taken as a tooth surface side, the tooth surface side is covered with a tooth cloth made of polyamide, such as nylon.
The tooth cloth made of polyamide, or the like, is impregnated with a rubber paste containing a filler, such as, carbon; or is impregnated with a mixed solution formed by mixing reactant between resorcinol and formaldehyde each with latex, that is vinyl-pyridine-styrene-butadiene-latex, chloroprene-latex or nitril-butadiene-latex, hereinafter referred to as "RFL solution".
One application of such a toothed belt is to drive a cam shaft of a car engine. In recent years, as the output of an engine has been increased, the operational environment of a toothed belt has become high in temperature and the transmission load has increased. On the other hand, there is a tendency that cars become necessaries for domestic use and become high-grade. The toothed belt has been prolonged in service life greater than the service life of the conventional toothed belt. To cope with the above-requirements, the toothed belt has been improved in the material of a tooth rubber and a back rubber, and in the material of a stretching member.
To improve the service life of a toothed belt, it becomes important to study the causes of the failure of the toothed belt. The main causes of the failure of the toothed belt lie in breakage of the belt and in chipping of teeth of the belt. The present invention is intended to overcome chipping of teeth of the belt, which is one of the main causes of failure of the belt.
As the general means for improving the chipping of teeth, there are both a means of improving the wear resistance of a tooth cloth and a means of improving the heat resistance of a tooth cloth. The means for improving the wear resistance of a tooth cloth includes means for increasing the amount of fibers per unit area of a tooth cloth, and a means of improving the wear resistance of fibers used for a tooth cloth. On the other hand, as the means of improving the heat resistance of a tooth cloth, there is a means in which, for a hydrogenated nitrile rubber, referred to as "HSN", a processing agent of an HSN rubber paste is used.
In the present invention, there has been studied how the wear, as a cause of chipping of teeth, proceeds in a toothed belt. To confirm the relationship between the presence, or absence, of a processing agent and wear resistance, a toothed belt, in which surfaces of teeth were covered with a polyamide made tooth cloth not processed by a processing agent, has been first evaluated, with a result that the surfaces of the teeth become nappy for a period of time from several to several tens minutes and the tooth cloth become cottony, thus leading to the chipping of the teeth. Therefore, it is considered that the tooth cloth, processed by a processing agent, is effective not only to be adhesively bonded on a tooth rubber, but also to be protected and to be bound in a suitable density by the processing agent. This enhances the wear resistance against chipping of teeth, resulting in improved durability.
However, it has become apparent that such a processing agent also accelerates wear. When a toothed belt is contacted with a pulley, a tooth cloth is slightly worn and, due to the wear of the tooth cloth, powder of the processing agent becomes present on the surface of the tooth cloth. The powder of the processing agent acts as abrasive when the toothed belt is contacted with the pulley. Specifically, a filler mixed in the processing agent for improving the wear resistance exhibits the reversed function, that is, acts to proceed the wear of the tooth cloth. In particular, the filler made of hard powder acts to abruptly process the wear.
An object of the present invention is to improve the wear resistance of a toothed belt and to enhance the service life against chipping of teeth by processing a tooth cloth constituting the toothed belt using a processing agent having a specified composition.
The above object is accomplished, according to the present invention, by providing a toothed belt in which surfaces of teeth of a tooth rubber are covered with a tooth cloth, characterized in that an impregnated layer by an RFL solution is formed on the tooth cloth and an impregnated layer by a rubber paste containing carbon in an amount of 10 phr, or less, relative to the polymer is formed on the RFL solution impregnated layer.
In addition, preferably, an amount of the solid content of the RFL solution stuck on the tooth cloth is in the range of from 3 to 20 wt % relative to the tooth cloth, and an amount of the solid content of the rubber paste stuck on the tooth cloth is in the range of from 6 to 18 wt % relative to the tooth cloth.
A tooth cloth is first impregnated with an RFL solution. The amount of the solid content of the RFL solution is in the range of 3 to 20 wt % relative to the tooth cloth. The tooth cloth is then impregnated with a rubber paste containing carbon in an amount of 10 phr relative to the polymer. The amount of the solid content of the rubber paste is in the range of from 6 to 18 wt % relative to the tooth cloth. The tooth cloth is thus sequentially formed with the impregnated layer by the RFL solution and the impregnated layer by the rubber paste. The surfaces of the teeth of tooth rubber are then covered with the tooth cloth.
When the toothed belt is contacted with a pulley, the tooth cloth is subjected to wear. The wear is mainly generated in the rubber paste impregnated layer. Because the carbon content in the rubber paste is 10 phr or less, even when the wear powder of carbon is present on the surfaces of teeth upon the contact between the toothed belt and the pulley, the wear powder has little effect as an abrasive, thus reducing the wear of the tooth cloth.